FY 2024

Magnitude, trends, and variability of the global ocean carbon sink from 1985 to 2018

DeVries, T., K. Yamamoto, R. Wanninkhof, N. Gruber, J. Hauck, J.D. Müller, L. Bopp, D. Carroll, B. Carter, T.-T.-T. Chau, S. Doney, M. Gehlen, L. Gloege, L. Gregor, S. Henson, J.H. Kim, Y. Iida, T. Ilyina, P. Landschützer, C. Le Quéré, D. Munro, C. Nissen, L. Patara, F. Perez, L. Resplandy, K. Rodgers, J. Schwinger, R. Séférian, V. Sicardi, J. Terhaar, J. Trinanes, H. Tsujino, A. Watson, S. Yasunaka, and J. Zeng

Global Biogeochem. Cycles, 37(10), e2023GB007780, doi: 10.1029/2023GB007780, View open access article at AGU/Wiley (2023)

This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985–2018, using a combination of models and observation-based products. The mean sea-air CO₂ flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr⁻¹ based on an ensemble of reconstructions of the history of sea surface pCO₂ (pCO₂ products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO₂, which is estimated at −2.1 ± 0.3 PgC yr⁻¹ by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr⁻¹ by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr⁻¹ of terrestrially derived CO₂, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO₂ products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr⁻¹ decade⁻¹, while biogeochemical models and inverse models diagnose an anthropogenic CO₂-driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr⁻¹ decade⁻¹, respectively. This implies a climate-forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate-driven variability exceeding the CO₂-forced variability by 2–3 times. These results suggest that anthropogenic CO₂ dominates the ocean CO₂ sink, while climate-driven variability is potentially large but highly uncertain and not consistently captured across different methods.

Plain Language Summary. The second REgional Carbon Cycle Assessment and Processes effort, or RECCAP2, provides a comprehensive assessment of global and regional greenhouse gas budgets. This paper focuses on the ocean carbon sink, and investigates the processes that control its magnitude, trends and variability. Observation-based techniques estimate that the net transfer of CO₂ from the atmosphere to the ocean, averaged over 1985–2018, is 1.6 billion tonnes of carbon per year, and that oceanic CO₂ uptake is increasing by 0.61 billion tonnes of carbon per year each decade. Models say that most of this CO₂ entering the ocean, and its increase over time, is driven by anthropogenic CO₂ emissions, which causes the ocean to take up 2.1–2.4 billion tonnes of carbon per year. There are some hints that climate change might be accelerating ocean carbon uptake, but the errors in our estimates are too large to know for sure right now. Our methods and observations will have to be improved in order to better detect the impact of climate change on the ocean carbon sink.